Method of producing toughened glass

ABSTRACT

Glass is toughened by quenching the hot glass in a bath which consists essentially of a major proportion of a carrying liquid which is inert relative to the glass and has a boiling point below the temperature of the hot glass which is to be quenched and of a minor proportion of a liquid having a boiling point at least 100* C. below the boiling point of the above mentioned carrying liquid. Thereby the speed of quenching the glass is greatly increased.

United States Patent Wartenberg 1 51 Apr. 4, 1972 [54] METHOD OF PRODUCING TOUGHENED GLASS [72] Inventor: Erwin W. Wartenberg, Brunnwiessen 6,

Stuttgart, Germany [22] Filed: Sept. 23, 1970 [21] Appl. No.: 74,897

Related US. Application Data [63] Continuation-impart of Ser. No. 692,778, Dec. 12,

I967, abandoned.

[30] Foreign Appllcatlon Priority Data Dec. 28, 1966 Germany ..W 43078 [56] References Cited UNITED STATES PATENTS 2,263,489 11/1941 Day s /11s #0 7552 0F PART/(4E: P59 c m 2 3,186,816 6/1965 Wartenberg ..65/ I I6 3 ,27 1,207 9/ l 966 Davis ..65/1 16 FOREIGN PATENTS OR APPLICATIONS 197,980 5/1958 Austria ..65/1 16 2,783 8/1874 Great Britain ..65/1 16 Primary Examiner-Arthur D. Kellogg Attorney-Snyder and Butrurn [57] ABSTRACT Glass is toughened by quenching the hot glass in a bath which consists essentially of a major proportion of a carrying liquid which is inert relative to the glass and has a boiling point below the temperature of the hot glass which is to be quenched and of a minor proportion of a liquid having a boiling point at least 100 C. below the boiling point of the above mentioned carrying liquid. Thereby the speed of quenching the glass is greatly increased.

30 Claims, 3 Drawing Figures Patehfed April 4, 1972 2 Sheets-Sheet l Fig. 3

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METHOD OF PRODUCING TOUGHENED GLASS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of co-pending application Ser. No. 692,778 filed Dec. 12, 1967 now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a method of producing toughened glass by quenching a hot glass body in a quenching bath which includes a quenching liquid.

It is known to increase the mechanical strength of glass by a toughening process. Such toughening is carried out by first heating the glass and then uniformly and suddenly cooling the heated glass, i.e., by quenching. Gases or liquids may be used as quenching agents. Toughened glasses find practical application, for example as glass windows for motor vehicles, due to the fact that toughened glass has increased strength and, furthermore, upon being subjected to stress the properly toughened glass sheet will fragment into a great number of relatively small particles which do not have sharp, and therefore dangerous, edges.

Known liquid quenching methods are effective for toughening glass sheets having a thickness of at least about 3.4 mm., to improve the mechanical strength of the glass which, upon severe impact, breaks into many relatively small and harmless particles. Control of particle count is essential in the manufacture of safety glass for motor vehicles.

Usually, safety glass used in motor vehicles has a thickness of between about 5 mm. and 6 mm. However, it is becoming more and more desirable to reduce the thickness of automobile glass, particularly in view of the larger glass area which is used in modern automobiles. Thus for economic as well as for structural reasons it has become important to reduce the thickness of the glass used as windows and windscreens in motor vehicles.

it has also been found that this safety-glass because of its high elasticity diminishes greatly the risk of severe skull injuries on head impact.

Many attempts have been made to produce extremely thin glasses, which in terms of their mechanical strength, and fracture characteristics and elasticity upon severe impact, are suitable for use in motor vehicles. It has been found desirable to reduce the thickness of automobile windows to between about 1.6 mm. and 2.5 mm. However, it has not been possible hitherto to toughen glass sheets of such small thickness by conventional methods, in such a manner that the toughened glass will comply with official safety requirements and will possess the mechanical strength desirable for automobile windows, windscreens and the like.

Recently chemical toughening methods have been developed for producing glass sheets having a thickness of between about 2.0 mm. and 2.5 mm. which sheets have a high degree of mechanical strength and form the desired large number of small particles upon being disintegrated by mechanical force. These methods are based on the principle of ion exchange. Although the glass sheets produced by this method are thinner and have higher modulus of rupture than glass which was toughened by the conventional quenching method, the ion exchange method has the great disadvantage that it cannot be used for toughening glass sheets of ordinary soda-lime-silica composition. Alumino-silicate glass must be used as an initial material in the ion-exchange process, in order to produce residual compressive stress at the surface of the glass by exchange of sodium for potassium or lithium ions. Toughening by ion exchange cannot be achieved with ordinary sheet glass because the chemical composition or ordinary sheet glass is unsuitable for ion exchange processes.

It is an object of the present invention to provide a process for the toughening of a wide range of glasses, irrespective of their composition, including ordinary sheet glass and, furthermore, to toughen such glass sheets which are relatively thin. By the method of the present invention it is possible to toughen glass sheets whose thickness is considerably less than 3.5 mm. and to obtain the same mechanical strength as is obtainable in alumino-silicate glasses by the ion exchange methods. Glass sheets having a thickness of only about 0.8 mm. when toughened in accordance with the present invention, will fragment, when shattered, into small and relatively harmless glass particles.

Toughened glass produced by the method of the present invention is particularly suitable as single sheet safety glass in motor vehicles. The mechanical strength of the toughened glass is about 8 times as high as the mechanical strength of untoughened glass and about three times the mechanical strength which is obtained by subjecting similar glass to a conventional quenching method.

The high degree of mechanical strength of the glass, particularly of glass sheets, is achieved by quenching the glass in a chilling liquid in such a manner that the glass will be cooled much faster than has been possible hitherto. The speed of cooling exceeds that which could be obtained by using pure water as a chilling liquid, although it has been assumed that pure water represents the fastest and most effective chilling liquid.

Many attempts have been made to toughen glass by quenching in water; however, these attempts were not successful because quenching of heated glass in water caused cracking of the glass or at least considerable damage to the glass surface so that water-quenched glasses had to be discarded.

US. Pat. No. 3,186,816 and German Pat. No. 1,034,333 disclose processes according to which heated glasses are quenched in a liquid and the energy required for vaporization of the liquid causes quick cooling of the glass. The liquid which is used for quenching is maintained by a succession of hot glasses at a temperature in the vicinity of and preferably slightly below, its boiling point.

It has been found possible by these prior methods to toughen glasses of thickness at least 3.5 mm. and to obtain thereby glasses which fragment into a great number of relatively small and harmless particles. However, it has not been possible to toughen glasses less than about 3.5 mm. thick, because liquids having the required high heat of vaporization, when used for quenching relatively thin glasses, cause immediate destruction of the glass. Furthermore, within a range of heat of vaporization between about and calories per gram, it was not possible to produce a substantial reduction of the length of the quenching period or a substantial increase in the speed of cooling of the glass and concomitant therewith an increase in the mechanical strength of the toughened glass.

The methods of the two patents mentioned above require the formation of a gas layer or vapor sleeve around the hot glass in order to obtain faster quenching without destruction of the glass. The patents describe special ways of enhancing the formation of such vapor sleeve. Surprisingly, it now has been found that, notwithstanding the fact that the cooling of the glass is caused by vaporization of quenching liquid, the formation of a continuous gas or vapor layer along the glass surface and the maintenance of such gas or vapor layer for a certain period of time is undesirable when toughening thin glass, because extremely quick cooling which is the decisive requirement for obtaining the desired high degree of toughening of thin glasses, is prevented by the formation of such vapor layer.

Consequently, the present invention proposes to maintain throughout the quenching process direct contact between hot glass and quenching liquid, without the production of an interposed vapor layer.

SUMMARY OF THE INVENTION According to the present invention, glass, particularly glass sheets of relatively small thickness, is toughened by being contacted with a quenching bath which consists essentially of a major proportion of a first liquid which is inert relative to the glass and has a boiling point below the temperature of the glass and of a minor proportion of a low boiling point liquid which has a boiling point significantly below the boiling point of the first liquid. The low boiling point liquid will come in direct contact with the surface of the hot glass sheet and will be caused to boil and will be vaporized while in direct contact with the hot glass surface, thereby causing rapid cooling of the glass due to the withdrawal from the hot glass of the heat of vaporization required for vaporizing the low boiling point li uid.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphic representation of the relationship with respect to various quenching, liquids and concentrations between heat of vaporization and the number of glass particles formed upon destructive impact;

FIG. 2 is a graphic representation of the relationship between concentration of two lower boiling point liquids, and the number of glass particles formed upon destructive impact; and

FIG. 3 is a graphic illustration of the relationship between the number of glass particles formed upon destructive impact and the concentration of a low boiling point liquid in the chilling liquid. 5

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, sudden cooling of the glass body is achieved due to vaporization of a low boiling point cooling liquid in direct contact with the surface of the hot glass body.

This is achieved by using as a liquid quenching medium, a chilling liquid which is a mixture of carrying liquid having a relatively high boiling point, but which is below the temperature of the heated glass body, and a second liquid which has a lower boiling point than the carrying liquid in an amount of up to about 4 percent of the weight of the chilling liquid thus formed. The low boiling point liquid must have a boiling point and heat of vaporization such that it will quickly evaporate when it is in contact with the hot glass. The low boiling point liquid is chosen in dependence on the degree of toughening required, and so that there is a relatively large difference, of at least 100 C., between the boiling point of the low boiling point liquid and the incipient boiling point of the carrying liquid, which is at least 200 C. The nature of the low boiling point liquid is such that upon vaporization it will not have any destructive effect on the surface of the hot glass body.

Due to the relatively small proportion of low boiling point liquid which is incorporated in the chilling liquid, the low boiling point liquid will be vaporized in contact with the glass surface but will not be capable of forming thereon a coherent gas or vapor layer on the glass surface.

The method of the present invention may be used to quench glasses of any composition, for instance, sodium-potassium glass or, generally, all types of alkali metal alkaline earth metal silicate glasses, e.g., soda-lime-silica glass. The method may be'used for the quenching of all types of plate glass and window glass as well as for special glasses such as boro-silicate glasses or lead-silicate glasses.

In an earlier liquid quenching method a piece of plate glass which has been heated nearly to its softening point, for example to about 630 C., and which is 1.8 mm. thick, and has an expansion coefficient of 90 X "C." was quenched in pure boiling carbontetrachloride and a continuous gaseous phase was formed on the surface of the cooling glass body and persisted for about 40 seconds. Glass which has been quenched in this manner has a mechanical strength which is about twice that of similar but untoughened glass. However, upon being subjected to destructive impact, the glass thus quenched will not disintegrate into a great number of relatively small and harmless particles.

The formation of a continuous vapor layer on the glass surface is prevented according to the present invention by incorporating in a carrying liquid whose incipient boiling point is at least 200 C., a relatively small proportion of a low boiling point liquid whose boiling point is at least 100 C. below the boiling point of the ca ying liquid. The chilling liquid thus formed is used as a quenching bath, preferably at a temperature which is in the vicinity of, usually slightly below,-the boiling point of the low boiling point liquid. Because the'boiling point of the carrying liquid is at least 100 C. higher than the boiling point of the low boiling point liquid, essentially only the low boiling point liquid is vaporized during the quenching of the hot glass. The presence in the chilling liquid of at least about 96 percent by weight of the higher boiling point canying liquid, prevents the formation of a continuous gas or vapor layer even though the low boiling point liquid is being vaporized and, consequently, the cooling and quenching of the glass takes place immediately the hot glass enters the liquid, by the vaporization of low boiling point liquid in direct contact with the hot glass surface. The entire heat required for the vaporization of the contacting low boiling point liquid is withdrawn rapidly from the glass and thereby very quick cooling is achieved during which essentially only low boiling point liquid is vaporized.

For obtaining the desired high degree of toughening, it is important that cooling of the glass from a temperature just below the softening point of the glass, i.e., from the temperature at which the hot glass is exposed to quenching, for example 630 C., down to a temperature below the strain point of the glass, e.g. about 350 C. is carried out as quickly as possible. Such very quick cooling is achieved according to the present invention by the vaporization of the low boiling point liquid in direct contact with the glass body. The further cooling of the glass from about 350- C. down to the temperature of the quenching bath is of little consequence in determining the final degree of toughening of the glass.

Referring now to the drawing, FIG. 1 illustrates the relation ship between heat of vaporization and the fragmentation number for carbon tetrachloride CCL; benzene C,I-I.; tertiary butanol (CI-I C-OH; ethanol C,I-I OH and methanol CH,OI-I employed as low boiling point liquid in a mineral oil Shell Q83 whose incipient boiling point is about 300 C. The lefthand curve represents a concentration of 0.3 percent by weight and the right-hand curve a concentration of 1 percent by weight. It is evident from the curves of FIG. 1 that with increasing heat of vaporization of the low boiling point liquid, the number of particles formed upon fragmentation and thus the degree of toughening of the glass will increase.

The marked points on the curves of FIG. I are selected from tests carried out under similar conditions with the samehigh boiling point liquid Shell 0.3.3, which has a density of 0.9 l viscosity of 17.5 cst at 100 C., and flash point of 290 C. The incipient boiling point of this oil is about 300 C.

Table I gives results obtained using carbon tetrachloride (boiling point 76 C.) as the low boiling point liquid.

A glass sheet whose coefficient of expansion is X 10." C. and dimensions X 100 X 1.8 mm. was heated in an electric furnace to about 630 C. and immediately thereafter was immersed in a quenching bath located below the furnace. The quenching bath of the chilling liquid was constituted by a mixture of 5 liters of the mineral oil with several grams of car.- bon tetrachloride, e.g. 9 grams of carbon tetrachloride is equivalent to 0.2 percent by weight of the chilling liquid; 22.5 grams of carbon tetrachloride is equivalent to 0.5 percent by weight of the chilling liquid; and 45 grams of carbon tetrachloride is equivalent to 1 percent by weight of the chilling liquid. a

The quenching bath was maintained at 100 C., and the hot Vegetable and animal fats oils and waxes including:

glass sheet upon immersion, was cooled within about 6 rape seed oil seconds to a temperature of about 200 C. The results Whale 9" achieved with different concentrations of carbon tetrachloride :2}: si were as follows: 5 shellac wax TABLE I CCliweight, percent 0.1 0.2 0.3 0.4 0.6 0.6 1.0 1.8 2.4 4.0 4.8 Fragmentation number.. 8 14 27 28 39 46 50 55 60 67 Modulus of rupturt (Xlll' p.s.i.) 30 37 39. 5 41. 2 42 44 45 47. 5 48. 5 47. 6 47 The process was repeated under the same conditions with hydrogenated Castor Oil the mineral oil Shell 0.8.3 as carrying liquid and with :zzx 300C methanol CH Ol-l (boiling point 68 C.) as the low boiling dodge: 216 point liquid. The results obtained were as follows: tridecane 243 tetradecane 253 TABLE II pentadecane 270 h d 287 011.011 0.1 0.2 0.4 0.5 0.0 1.0 305 Fragmentation number 32 50 60 64 62 72 Modulus of rupture (x10 p.s.i.) as 43 4s 51 50.5 65 Elma 343 W WAV naphthalene 2|0 'w'wirfi "4? W l-allyl-naphthalerle 256 In these tests 4.5 grams of methanol were added to 5 liters l-chloro-naphthalene 263 of the mineral oil to give a concentration of 0.1 percent by l-chlflw-naphlhfllme 256 weight; and 22.5 grams of methanol were added to 5 liters of :3; the mineral oil to give a concentration of 0.5 percent by ,:3 dichlm napmhalenc weight. l,7-dichloro-naphthalene 235 These results are illustrated graphically in FIGS. 2 and 3, f zfli z i r h h I is? and FIG. 2 shows that when between 0.2 percent and 1 perz grggz 280 cent by weight of carbon tetrachloride is added to the mineral 123,448, hydmnapmhalene oil the chilling liquid thus formed is heated to between about (tetraline) 207 80 C. and 100 C. and used at that temperature as quenching f 325 l-chloro-5mum-naphthalene 360 bath for toughening glass 1.8 mm. thick, by heating the glass ln omchloronapmhalme M0 conventional manner and then quenching the glass in the bath, m wanna 289 toughened glass is produced which, when subjected to l-lelralane 2 diphenyl 255 destructive impact, has a fragmentatlon number of between 04" hen I 332 about 15 and 45 particles per square centimeter and has a WRSPMHYYI 36s modulus of rupture of between 14 X 10 p.s.i. and 28 X 10 diamyl-benzene 260-280 p.s.i. or even higher, up to 42 X 10 p.s.i. lriamyl benzene 0-320 As is evident from the curves of FIG. 2, the fragmentation 32 number and thus the tensile strength of the glass increases mchlombemene 210 with increasing concentration of low boiling point liquid. The orthophcsphoric acid 2l3 right-hand curve of FIG. 2 shows that substantially the same "il i p c s :2:

- trllso uty p osp orlc ester trend is lndlcated when methanol ls used. mphenyl phosphoric ester 245 FIG. 3 is a curve, based on some of the results tabulated 1n mpmpyl phosphoric 252 Table l, in which fragmentation number on destructive impact tris(3,5-dimethyl-phenyl) is plotted against the concentration of carbon tetrachloride in tn (Z-tulyl) phosphorlc ester 4l0 the mineral oil. It is evident from thls curve that when the conhexadecane 287 centration of carbon tetrachloride is about 1.5 percent by Lmmdecyne 234 weight there is only further slight increase in the degree of hexadecyl ester 360 toughening of the glass for a considerable increase in the con- 'f" hexadecanolc methyl ester 415 centration of carbon tetrachloride. glycerol 'ripalmhale 3'0 Table l and FIG. 3 show that the degree of toughening of the stearin 300 glass reaches a maximum represented by a fragmentation phenyls tearate 267 number of about particles per cm.", when the concentra- 3 :22: tion of carbon tetrachloride is about 2.4 percent to 3 percent 55 chum: 330 by weight. Thereafter as the concentration is increased the l-octadecyne 313 degree of toughening gradually falls, and when the concentra- 272: h 355 tion is increased to above 4 percent by weight a pronounced g'lodzhilhmcene reduction in the strength of the toughened glass is observed. 91mm amhmcene M7 Furthermore it has been found that too great a concentration A ve wide choice of 1 l 5 any s the bmlmg 4 pememb'y may f ossiblef as exem lified wil I? li liit i s 2: 21141.32 result in the production of surface defects in the glass during 2 waxes compgunds with confiensed gel-Zane rings paraf quenching, e.g. the formation of grooves and hairline cracksln r ammatic phosphoric acid esters and p acid the glass Snrf.ace' glycerol ester. Usually the carrying liquid is chosen in depen- Generally in carrying out the lnventlon all liquids with hlgh d h h th heat of va rization and boiling points high relative to that of ence on t e temperature at w w e quenc mg bath to be P? a used, so that the bath temperature is well below the boiling the low bolllng point liquid, and preferably above 200 C. and point ofthe y g liquid 0 V i 400 g j i gg a i gg z gg The choice of the higher boiling point carrying liquid is of g a e I? ;:22 y in n uid ma be an relatively little importance in determining the speed of cooling c e .ects 8 2185.5 if 3 g q y 7 of the glass and the degree of toughening which is achieved by i g an q g .31 t d below b wa ofexam le the quenching of the glass, since the main heat transfer from g. a o g p the glass is achieved by the vaporization of the low boiling W1 t 01 mg pom m w 6 p point liquid which forms only a small proportion of the chilling Mineral oils liquid F l '1 incl ient boiling point about Li lin a non oils 300 c. or above It will be seen, as lllustrated ln FIG. I, that by using the same Kerosene 200*- 300C. first carrying llquld 1n comblrlatlon wlth equal proportions of various low boiling point liquids, the speed of cooling of the glass and thereby the number of particles per square centimeter which are formed upon fragmentation and the mechanical strength of the glass, increases with increasing heat of vaporization of the low boiling point liquid. This shows that the rapid cooling of the glass is actually achieved by the vaporization of the low boiling point liquid.

Suitable low boiling point liquids which are to be mixed in proportion of up to about 4 percent with the carrying liquid, should be chemically inert with respect to the glass to be quenched and include for example methanol, ethanol, carbon tetrachloride, toluene, trichloroethylene, benzene, monochlorobenzene, and chloroform. Preferably the boiling point of the low boiling point liquid should be between about 70 C. and 150 C., and the low boiling point liquid chosen is a liquid whose boiling point is at least 100 C. below the incipient boiling point of the particular carrying liquid.

It is advantageous for producing highly toughened glass to choose lower boiling as well as higher boiling liquids which have a relatively high heat of vaporization.

Results of tests carried out with. some other low boiling point liquids will now be indicated in Tables III to VII.

TABLE III Monochlorobenzene C H Cl (boiling point 132 C.) was mixed with the same mineral oil Shell 0.8.3." The glass was heated to about 720 C. and the quenching bath was at 100 C.

C.H,Cl weight k 0.5 0.6 0.8 1.0 Fragmentation 19 20 35 30 Number TABLE IV Ethanol C,H,,OI-l (Boiling point 77.6 C.) was mixed with Shell 0.8.3 oil and the quenching bath was maintained at 100 C.

canon weight k 0.2 0.4 0.5 1.0 Fragmentation 53 63 66 68 Number I TABLE V Chloroform CI-ICl: (Boiling point 61 C.) was mixed with Shell 0.B.3 oil and the quenching bath was maintained at 100 C.

CHCI, weight 5 0.7 L 1.5 2

Fragmentation Number 30 40 54 60 TABLE VI Benzene C ll (Boiling point 80 C.) was mixed with Shell 0.8.3" oil and the quenching bath was maintained at 80 C.

CJ'I. weight a. 0.2 0.3 0.5 1.0

Fragmentation Number 21 20 46 52 TABLE VII Trichloroethylene C,H Cl, (Boiling point 86 C.) was mixed with Shell 0.8.3" oil and the quenching bath was maintained at'lOO" C.

C,H Cl, weight k 0.4 1.0 1.5 2 Fragmentation Number 14 35 52 56 When low boiling point liquids are used which contain OH groups, and have a high heat of vaporization, e.g. CHaOH, C,,H,Ol-l, toughened glasses are obtained which may have a modulus of rupture of about 42,000 p.s.i. and thus are far superior to similar glasses which were quenched in conventional manner and thereby obtained a modulus of ruptugegf about 14,000 p.s.i. The values for modulus of rupture which are achieved according to the present invention are of the same magnitude as those of glasses which had been toughened by the ion exchange method. The number of particles into which toughened glasses will disintegrate upon destructive impact may be more than l00/cm. even if the glass was of a thickness of only 1.5 mm. The mechanical strength of such very thin glasses which are toughened by the method of the present invention makes it possible to bend these glasses easily so that the glass originally may be formed as a flat sheet and subsequently may be installed in curved rigid frames because it is able to adjust itself to a certain degree of curvature. The

method of the present invention may also be used for toughening hollow or curved glass. Due to the high rate of cooling the method is particularly suitable for glass having a relatively small coefficient of expansion. Thus, good results are obtained with glasses having coefficients of expansion of between 5 X 10' "CF and X 10' "CF and preferably between 30 X 10' C7 and 90 X 10 C.".

As pointed out above, the proportion of low boiling point liquid in the chilling liquid should not exceed about 4 percent by weight of the chilling liquid and generally will be between about 0.1 percent and 4 percent by weight, preferably between 0.5 percent and 2 percent.

It is a significant advantage of the present invention that by changing the percentage of the low boiling point liquid in the chilling liquid within the above indicated ranges the fragmentation number, or the degree of toughening, may be adjusted as desired.

Generally, in order to obtain the same results, the smaller the expansion coefiicient of the glass, the higher must be the heat of vaporization of the low boiling point liquid, or, at the same heat of vaporization, the concentration of low boiling point liquid must be higher.

The temperature of the quenching bath is significant, and is usually maintained at about, often just below, the boiling point of the low boiling point liquid. This ensures immediate boiling of the low boiling point liquid on the glass surface and very rapid heat extraction from the glass surfaces as the glass enters the chilling liquid.

Table VI]! indicates the way in which variation of the bath temperature can be seen to alter the degree of toughening of the glass.

In these experiments, 0.5 percent by weight dichlorobenzene C d-1 C], (Boiling point 180 C.) were mixed with Shell 0.3.3 oil to constitute the chilling liquid.

TABLE Vlll Bath Temperature I00 C I50 C 200 C. Fragmentation Number l3 17 20 Thus a higher degree of toughening is achieved as the bath temperature approaches the boiling point of the added dichlorobenzene, and when the bath temperature exceeds'the boiling point of the dichlorobenzene an even greater degree of toughening results, although further bath temperature increase would not be advisable because it would increase vaporization of the dichlorobenzene.

Suitable low boiling point liquids are as follows, the list including those liquids already specifically referred to and the boiling points being given in C:

v ethyl isobutyl ether 81 l-chloroethyl ether 72 Z-chlorocthyl ether 92 benzene 80 l-chloro-Z methoxybenzene 9O ethane phosphoric dimethylester 82 monochlorobenzene I32 dichlorobenzerie I80 chloroform 6i Heptane 98 Hexane 69 Octane I26 I, l ,l ,2-tetrachloroethane [30 l, l ,2,2-tetrachlnroethane I46 l,l,2, trichloroethane !l3 2-chlorol -phenylethanol I28 ternary bulyl alcohol 82 2,2dichloroethano| 146 Z-dimethyl-arnino ethanol l35 glycol monoethyl ester 135 tetrachloroethene 121 butyl propyl ether ll7 S-methyl butyl ether 130 cyclohexyl methyl ether 133 dibutyl ether I42 2-chlorodiethyl ether I07 1,2 -dichlorodiethyl ether I40 2'-chloro ethenyl ether 108 l,2-dichloroethenyl ether I28 ethyl hexyl ether !42 Toluene l I phosphorus oxychloride I phosphorus thiochloride l25 phosphorus trimethyl ester ll 1 isobutoxyethanol I59 ethenyl phenyl ether I55 2-chloro-toluene I59 3-chloro-toluene 162 4-chloro-toluene I62 2-butoxy-ethanol l7l benzyl methyl ether I74 2,2-dichlorodiethyl ether benzyl chloride When carrying out the method of the present invention on a large scale, such as for the mass production of glasses for automobiles, it is advantageous to replenish continuously the vaporized low boiling point liquid. This may be accomplished either by condensing the vapors of low boiling point liquid which rise from the quenching bath by indirect heat exchange, for instance by means of cooling coils and recycling the condensed liquid into the quenching bath, or by the feeding of small amounts of low boiling point liquid, for example continuously drop-by-drop, equivalent to the amounts thereof which have been vaporized.

The rapid cooling of the hot glass which is achieved by the method of the present invention is based on the vaporization of the low boiling point liquid which is present in a relatively low concentration in the chilling liquid. The rapid withdrawal of heat from the glass by means of the liquid directly contact ing the glass could also be carried out by decomposition or chemical conversion of a liquid, and all energy consuming processes in which the glass is rapidly cooled when it enters the chilling liquid can be effective in producing the required degree of toughening of the glass.

The vaporizing or decomposing cooling liquid is preferably continuously replaced at such a rate that the concentration of Phenyl stearate, 267 0. Cyclohexanol, 160 0.

Paraffin wax, 300 0.- glyl'clollieiigg gm C.

eno Hexadecaml' C "{genziyl alcdoho1], 2(()}5 c.

. cet c aci 1 8 steam: acid 358 C {Trifluoroacetic acid, 72 C.

Octane, 126 C. Stearin Heptane, 98 C.

, Trichloroethylene, 74 C. Octachloronaphthalene, 440 O iglshqrcggluene, 162 0. (bath at Hexadecanoic methyl ester, {Furlura iEetate: 175 0.

Glycol diacetate, 180190 0.

Carbon tetrachloride, 76 0. Mineral oil, 30 C Trichloroethylene, 74 C.

Monochlorobenzene, 132 C. Tetraline, 207 C Monochlorobenzene, 132 C.

Orthophosphoric acid, 213 C Phosphorus oxychloride, 105 C.

low boiling point liquid in the chilling at least the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalent of the following claims.

I claim:

1. In the method of toughening glass which includes quenching glass with a quenching liquid from a temperature at about the softening point of said glass rapidly to a temperature below the strain point temperature of said glass, the improvement which comprises the steps of:

a. forming a body of carrying liquid which is inert to said glass and which carrying liquid has a selected boiling point less than said strain point temperature of said glass;

. forming said quenching liquid by providing a component in said body of carrying liquid which component is present in minoramount in said quenching liquid and which component has a boiling point significantly less than said selected boiling point of the carrying liquid whereby said component will vaporize in preference to saidcarrying liquid, which minor amount of said component and which difference in boiling points of said carrying liquid and said component co-operatively are sufiicient (l) to maintain throughout the quenching process direct contact between the hot glass and quenching liquid without the production of an interposed vapor layer and (2) to cool the glass surface instantaneously by a significant amount producing an initial surface-to-center temperature gradient through the glass thickness sufficient to produce on more gradual cooling effected thereafter a desired ultimate degree of toughening of the glass; and

. maintaining the quenching liquid of step (b) at a temperature less than the boiling point of said carrying liquid.

2. In the method of toughening glass to provide a modulus ofrupture therefor in the range 14 X 10 p.s.i. to 42 X 10 p.s.i. and a fragmentation number on fracture of between 8 and particles per square centimeter, which includes quenching glass with a quenching liquid from a temperature at about the softening point of said glass rapidly'to a temperature below the strain point temperature of the glass, the improvement which comprises the steps of: e

a. forming a body of carrying liquid which is inert to said glass and which carrying liquid has a selected boiling point less than the strain point temperature of the glass;

. forming said quenching liquid by providing a component in said body of carrying liquid which component is present in minor amount in said quenching liquid and which component has a boiling point significantly less than said selected boiling point of the carrying liquid whereby said component will vaporize in preference to said carrying liquid, which minor amount of said component and which difference in boiling points of said car- 'rying liquid and said component co-operatively are sufficient (l) to maintain throughout the quenching process direct contact between the hot glass and quenching liquid without the production of an interposed vapor layer and (2) to cool the glass surface instantaneously by a significant amount producing an initial surface-to-center temperature gradient through the glass thickness sufficient to produce on more gradual cooling effected thereafter a desired ultimate degree of toughening of the glass; and maintaining the quenching liquid of step (b) at a temperature less than the boiling point of said carrying liquid.

3. A method according to claim 1, wherein the temperature of the quenching liquid is maintained at about the boiling point of said component.

4. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is an oil whose boiling range begins at 300 C, with up to 4 percent by weight of said component which is a low boiling point liquid whose boiling point is at least 100 C below the bottom of said boiling range.

5. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a liquid whose incipient boiling point is in the range of 200 C to 400 C, and up to 4 percent by weight of said component which is a low boiling point liquid whose boiling pointis in the range of 70 C to 150 C.

6. A method according to claim 1, wherein said component is added to the quenching liquid to maintain a substantially y constant concentration thereof in the quenching liquid.

7. A method according to claim 1, wherein the heat of vaporization of said component is above 40 cal. g.'.

8. A method according to claim 1, wherein the heat of vaporization of said component is between 40 cal. g? and 300 cal. g..

9. A method according to claim 1, wherein said quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C, and said component which is carbon tetrachloride.

10. A method according to claim 1, wherein said quenching liquid consists of a major proportion'of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C, and said component which is trichloroethylene.

11. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C., and said component which is methanol.

12. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C., and said component which is toluene.

13. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C, and said component which is chlorobenzene.

14. A method according to claim 13, wherein said component is monochlorobenzene.

15. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300 C., and said component which is phosphorous oxychloride.

16. A method according to claim 1, wherein the quenching liquid includes from 0.1 percent to 4 percent by weight of said component.

17. A method according to claim 1, wherein the quenching liquid includes from 0.1 to 4 percent by weight of said component which is a liquidcontaining -OH groups.

18. A method according to claim 1, w erem said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a mineral oil having an incipient boiling point in the range of 200 C. to 400 C.

19. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a wax having an incipient boiling point in the range 200 C. to 400 C.

20. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a compound with condensed benzene rings having an incipient. boiling point in the range 200 C. to 400 C.

21. A method according to claim 18, wherein compound is terphenyl.

22. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a paraffin having an incipient boiling point in the range 200 C. to 400 C.

23. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is orthophosphoric acid.

24. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is an aromatic phosphoric acid ester having an incipient boiling point in the range 200 C. to 400 C.

25. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a palmitic acid-glycerol ester.

26. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is stearin.

27. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is tetraline.

28. A method according to claim 26, wherein said component is trichloroethylene.

29. A method according to claim 27, wherein said component is monochlorobenzene.

30. A method according to claim 23, wherein said component is phosphorus oxychloride. 

2. In the method of toughening glass to provide a modulus of rupture therefor in the range 14 X 103 p.s.i. to 42 X 103 p.s.i. and a fragmentation number on fracture of between 8 and 100 particles per square centimeter, which includes quenching glass with a quenching liquid from a temperature at about the softening point of said glass rapidly to a temperature below the strain point temperature of the glass, the improvement which comprises the steps of: a. forming a body of carrying liquid which is inert to said glass and which carrying liquid has a selected boiling point less than the strain point temperature of the glass; b. forming said quenching liquid by providing a component in said body of carrying liquid which component is present in minor amount in said quenching liquid and which component has a boiling point significantly less than said selected boiling point of the carrying liquid whereby said component will vaporize in preference to said carrying liquid, which minor amount of said component and which difference in boiling points of said carrying liquid and said component co-operatively are sufficient (1) to maintain throughout the quenching process direct contact between the hot glass and quenching liquid without the production of an interposed vapor layer and (2) to cool the glass surface instantaneously by a significant amount producing an initial surface-to-center temperature gradient through the glass thickness sufficient to produce on more gradual cooling effected thereafter a desired ultimate degree of toughening of the glass; and c. maintaining the quenching liquid of step (b) at a temperature less than the boiling point of said carrying liquid.
 3. A method according to claim 1, wherein the temperature of the quenching liquid is maintained at about the boiling point of said component.
 4. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is an oil whose boiling range begins at 300* C, with up to 4 percent by weight of said component which is a low boiling point liquid whose boiling point is at least 100* C below the bottom of said boiling range.
 5. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a liquid whose incipient boiling point is in the range of 200* C to 400* C, and up to 4 percent by weight of said component which is a low boiling point liquid whose boiling point is in the range of 70* C to 150* C.
 6. A method according to claim 1, wherein said component is added to the quenching liquid to maintain a substantially constant concentration thereof in the quenching liquid.
 7. A method according to claim 1, wherein the heat of vaporization of said component is above 40 cal. g.
 1. 8. A method according to claim 1, wherein the heat of vaporization of said component is between 40 cal. g. 1 and 300 cal. g.
 1. 9. A method according to claim 1, wherein said quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C, and said component which is carbon tetrachloride.
 10. A method according to claim 1, wherein said quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C, and said component which is trichloroethylene.
 11. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C., and said component which is methanol.
 12. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C., and said component which is toluene.
 13. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C, and said component which is chlorobenzene.
 14. A method according to claim 13, wherein said component is monochlorobenzene.
 15. A method according to claim 1, wherein the quenching liquid consists of a major proportion of said carrying liquid which is a mineral oil having an incipient boiling point of about 300* C., and said component which is phosphorous oxychloride.
 16. A method according to claim 1, wherein the quenching liquid includes from 0.1 percent to 4 percent by weight of said component.
 17. A method according to claim 1, wherein the quenching liquid includes from 0.1 to 4 percent by weight of said component which is a liquid containing -OH groups.
 18. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a mineral oil having an incipient boiling point in the range of 200* C. to 400* C.
 19. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a wax having an incipient boiling point in the range 200* C. to 400* C.
 20. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a compound with condensed benzene rings having an incipient boiling point in the range 200* C. to 400* C.
 21. A method according to claim 18, wherein compound is terphenyl.
 22. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a paraffin having an incipient boiling point in the range 200* C. to 400* C.
 23. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is orthophosphoric acid.
 24. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is an aromatic phosphoric acid ester having an incipient boiling point in the range 200* C. to 400* C.
 25. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is a palmitic acid-glycerol ester.
 26. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is stearin.
 27. A method according to claim 1, wherein said quenching liquid consists of 0.1 to 4 percent by weight of said component which is a low boiling point liquid and 96 to 99.9 percent by weight of said carrying liquid which is tetraline.
 28. A method according to claim 26, wherein said component is trichloroethylene.
 29. A method according to claim 27, wherein said component is monochlorobenzene.
 30. A method according to claim 23, wherein said component is phosphorus oxychloride. 